...a baseball bat.是什么意思？

...a baseball bat.

棒球球棒

baseball

n. 棒球，[体]棒球运动；

例句
Two boys were in the street, tossing a baseball back and forth.
两个男孩在街上来来回回地投接棒球。

bat

n. 蝙蝠，短棍，（棒球等的）球棒，（网球等的）球拍，白话，口语，成语，俗话，（口）打击，猛击，（海上）导弹；vt.& vi. 用球板击球，用球棒击球（尤指板球或棒球运动）；

例句
The old judge doesn't like the thought of no one going in to bat for the accused.
老法官不希望没有人为被告辩护。